The relationship between magnetic susceptibility and the ferromagnetic mineral content of rock varies with mineral assemblage, rock type, and with grain size, shape and orientation, but in all cases there has been found to be a strong, sometimes nearly linear correlation between the two. The magnetic susceptibility of magnetic rocks that contain ore-grade concentrations of iron minerals is high, making it possible to obtain borehole measurements of good quality with relatively simple, low-sensitivity logging probes. Measurements become increasingly difficult as the ferromagnetic mineral content decreases to the low levels generally associated with sedimentary rocks (&lt;0.1 percent magnetite). In order to measure the magnetic susceptibility of these rocks, the sensitivity of the well-logging system must be increased several orders of magnitude, and consequently noise and drift due to temperature variation and mechanical stress of components in the measurement system became significant. Nevertheless, making borehole measurements of the magnetic susceptibility of rocks that have low concentrations of ferromagnetic minerals is desirable because these measurements may reveal alteration zones associated with the emplacement of valuable non-ferrous minerals, particularly uranium in roll-type deposits.
To provide borehole measurement of the magnetic susceptibility of rock, well-logging probes have been developed which include sensing elements consisting of a solenoid wound on a cylindrical core made of a high-permeability ferromagnetic material. The coil is connected in the inductance arm of a Maxwell bridge, and variations in the off-balance output signal of the bridge are used to detect variations in the magnetic susceptibility of rock surrounding the probe in a borehole.
Changes in the magnetic susceptibility of rock are accompanied by nearly proportional changes in the amplitude of the quadrature phase of the bridge output signal of the probe. It was found, however, to be extremely difficult to stabilize the drift and reduce the noise so that meaningful borehole-logging measurements could be made at high sensitivities with uncompensated probes.
In order to stabilize drift caused by temperature variations, probes are designed with a temperature regulated electric heater which surrounds the sensing coil or some other portion of the probe. Prior art temperature stabilized probes are shown in U.S. Pat. Nos. 2,640,869 to C. W. Zimmerman and 3,818,323 to D.J. Dowling et al. These designs represent a significant contribution over previous unstabilized probe constructions, but some significant factors still exist which adversely affect the accuracy of measurements accomplished with the prior art probes. For example, electrical power sent down a well-logging cable is dependent upon cable temperature, resistance, and probe operating conditions. Even a well regulated voltage applied to the cable at the surface will arrive at the downhole electronics diminished by "IR" voltage drops caused by the current (I) being demanded downhole passing through cable wires which have resistance (R). As less current is needed downhole, more voltage will arrive downhole because the IR voltage drop will be lower. Therefore, voltage delivered downhole must be considered as unregulated and, depending on cable resistance (usually larger for long cables or cables having more, smaller wires), can vary over a wide range. For example, a current of 250 milliaps that might be required by a probe heater would cause a voltage drop of 100 volts over a cable having 200 ohms each in the power supply ad return wires needed for borehole work down to 8000 feet. A variation of .+-.50 milliamps about this current would cause the IR voltage drop to vary by .+-.10 volts.
In an attempt to compensate for the significant voltage variations caused by cable temperature, cable resistance and varying probe operating conditions, probe systems have been designed with two separate power supplies; a positive and a negative supply. Such probe systems, maufactured by Simplec Manufacturing Company of Dallas, Texas, are much improved over previous systems, but sill experience drift and noise which make accurate and reliable measurements of magnetic susceptibility in rocks with low concentrations of magnetic materials difficult to obtain.